Methods for etching carbon nano-tube films for use in non-volatile memories

ABSTRACT

Memory cells, and methods of forming such memory cells are provided that include a steering element coupled to a carbon-based reversible resistivity-switching material. In particular embodiments, methods in accordance with this invention etch a carbon nano-tube (“CNT”) film formed over a substrate, the methods including coating the substrate with a masking layer, patterning the masking layer, and etching the CNT film through the patterned masking layer using a non-oxygen based chemistry. Other aspects are also described.

CROSS-REFERENCE TO RELATED APPLICATIONS

This application claims the benefit of U.S. Provisional PatentApplication Ser. No. 61/044,314, filed Apr. 11, 2008, and titled“Methods For Etching Carbon Nano-Tube Films For Use In Non-VolatileMemories,” which is hereby incorporated by reference herein in itsentirety for all purposes, and claims the benefit of U.S. ProvisionalPatent Application Ser. No. 61/081,029, filed Jul. 15, 2008, and titled“Methods For Etching Carbon Nano-Tube Films,” each of which is herebyincorporated by reference herein in its entirety for all purposes.

TECHNICAL FIELD

This invention relates to non-volatile memories, and more particularlyto a memory cell that includes a carbon-based memory element, andmethods of forming the same.

BACKGROUND

Non-volatile memories formed from reversible resistance-switchingelements are known. For example, U.S. patent application Ser. No.11/968,154, filed Dec. 31, 2007, titled “MEMORY CELL THAT EMPLOYS ASELECTIVELY FABRICATED CARBON NANO-TUBE REVERSIBLE RESISTANCE-SWITCHINGELEMENT AND METHODS OF FORMING THE SAME” (the “'154 application”), whichis hereby incorporated by reference herein in its entirety for allpurposes, describes a rewriteable non-volatile memory cell that includesa diode coupled in series with a carbon-based reversibleresistivity-switching material such as carbon.

However, fabricating memory devices from rewriteableresistivity-switching materials is technically challenging, and improvedmethods of forming memory devices that employ resistivity-switchingmaterials are desirable.

SUMMARY

In a first aspect of the invention, a method of etching a carbonnano-tube (“CNT”) film formed over a substrate is provided, the methodincluding coating the substrate with a masking layer, patterning themasking layer, and etching the CNT film through the patterned maskinglayer using a non-oxygen based chemistry.

In a second aspect of the invention, a method of forming a memory cellis provided, the method including forming a layer of a CNT materialabove a substrate, and etching the CNT material in a plasma etch chamberusing boron trichloride (BCl₃) and dichlorine (Cl₂) and a substrate biaspower of between about 50 and about 150 Watts.

In a third aspect of the invention, a method of forming a memory cell isprovided, the method including forming a layer of a CNT material above asubstrate, and using a non-oxygen based chemistry to etch the CNTmaterial to have nearly vertical sidewalls and little or no undercut ofthe CNT material.

Other features and aspects of this invention will become more fullyapparent from the following detailed description, the appended claimsand the accompanying drawings.

BRIEF DESCRIPTION OF THE DRAWINGS

Features of the present invention can be more clearly understood fromthe following detailed description considered in conjunction with thefollowing drawings, in which the same reference numerals denote the sameelements throughout.

FIG. 1 depicts a cross-sectional, elevational schematic diagram of anexemplary memory cell in accordance with an embodiment of the presentinvention, the memory cell comprising a sidewall liner surrounding ametal-insulator-metal structure.

FIGS. 2A and 2B depict elevational cross-sections of other exemplarymemory cells in accordance with embodiments of the present invention,each memory cell comprising a sidewall liner surrounding ametal-insulator-metal structure in series with a diode.

FIG. 3 is a perspective view of an exemplary memory level of amonolithic three dimensional memory array provided in accordance withthe present invention.

DETAILED DESCRIPTION

Certain carbon-based films, including but not limited to CNTs, graphene,amorphous carbon containing microcrystalline or other regions ofgraphene, and other graphitic carbon films, etc., may exhibitresistivity switching properties that may be used to formmicroelectronic non-volatile memories. Such films therefore arecandidates for integration within a three-dimensional memory array. Forinstance, CNT materials have demonstrated memory switching properties onlab-scale devices with a 100× separation between ON and OFF states andmid-to-high range resistance changes. Such a separation between ON andOFF states renders CNT materials viable candidates for memory cellsformed using the CNT materials in series with vertical diodes, thin filmtransistors or other steering elements.

In the aforementioned example, a metal-insulator-metal (“MIM”) stackformed from a carbon-based material sandwiched between two metal orotherwise conducting layers may serve as a resistance change materialfor a memory cell. Moreover, a carbon-based MIM stack may be integratedin series with a diode or transistor to create a read-writable memorydevice as described, for example, in the '154 application.

However, when CNT material is used to form a memory cell, the depositedor grown CNT material often has a rough surface topography, withpronounced thickness variations, such as numerous peaks and valleys. Therough surface topography can cause difficulties in forming a memorycell. For example, the rough surface topography of CNT material can makeCNT materials difficult to etch without excessive etching of theunderlying substrate, increasing fabrication costs and complexityassociated with their use in integrated circuits.

Exemplary methods in accordance with this invention form a memory cellthat includes a memory element formed from CNT material. In particular,the CNT material may be etched using a plasma etcher and BCl₃ and Cl₂chemistries under relatively low bias conditions (e.g., about 100 W).CNT material etched using such techniques have been observed to havenearly vertical sidewalls and little or no undercut of the CNT material.

In at least some embodiments, pure CNTs may be deposited by CVD growthtechniques, colloidal spray on techniques, and spin on techniques.Additionally, carbon material deposition methods may include, but arenot limited to, sputter deposition from a target, plasma-enhancedchemical vapor deposition (“PECVD”), PVD, CVD, arc discharge techniques,and laser ablation. Deposition temperatures may range from about 300° C.to 900° C. A precursor gas source may include, but is not limited to,hexane, cyclo-hexane, acetylene, single and double short chainhydrocarbons (e.g., methane), various benzene based hydrocarbons,polycyclic aromatics, short chain ester, ethers, alcohols, or acombination thereof. In some cases, a “seeding” surface may be used topromote growth at reduced temperatures (e.g., about 1-100 angstroms ofiron (“Fe”), nickel (“Ni”), cobalt (“Co”) or the like, although otherthicknesses may be used).

In some embodiments, the carbon-based resistivity-switching material maybe composed of amorphous carbon or a dielectric filler material mixedwith graphitic carbon, deposited in any of the above mentionedtechniques. A particular embodiment of this integration scheme includesa spin or spray application of the CNT material, followed by depositionof amorphous carbon from an Applied Materials, Inc., Producer™ tool foruse as carbon-based liner material. The optional carbon-based protectiveliner can be deposited using a deposition technique similar to ordifferent than that used to deposit the CNT material.

The carbon-based resistivity-switching material may be deposited in anythickness. In some embodiments, the carbon-based resistivity-switchingmaterial may be between about 1-1000 angstroms, although otherthicknesses may be used. Depending on device construction, such asdescribed herein, preferred ranges may include 200-400 angstroms,400-600 angstroms, 600-800 angstroms, and 800-1000 angstroms.

Exemplary Embodiments

In accordance with a first exemplary embodiment of this invention,formation of a microelectronic structure includes formation of an MIMdevice having a carbon film disposed between a bottom electrode and atop electrode, the carbon film comprising, for instance, aresistivity-switchable CNT layer. The structure also includes adielectric sidewall liner provided to protect the carbon-based materialfrom degradation during a dielectric fill step.

FIG. 1 is a cross-sectional elevational view of a first exemplarymicroelectronic structure 100, also referred to as memory cell 100,provided in accordance with this invention. Memory cell 100 includes afirst conductor 102 formed over a substrate (not shown), such as over aninsulating layer over the substrate. First conductor 102 may include afirst metal layer 104, such as a tungsten (“W”), copper (“Cu”), aluminum(“Al”), gold (“Au”), or other metal layer. First conductor 102 maycomprise a lower portion of an MIM layerstack structure 105 and functionas a bottom electrode of MIM 105. An adhesion layer 106, such as atungsten nitride (“WN”), titanium nitride (“TiN”), tantalum nitride(“TaN”), molybdenum (“Mo”), or similar layer, is optional but is shownin FIG. 1 formed over first metal layer 104. In general, a plurality ofthe first conductors 102 may be provided and isolated from one another(e.g., by employing silicon dioxide (“SiO₂”) or other dielectricmaterial isolation between each of first conductors 102). For instance,first conductor 102 may be a word-line or a bit-line of grid-patternedarray.

A layer of CNT material 108 is formed over first conductor 102 using anysuitable CNT formation process. CNT material 108 may comprise a middleportion of MIM layerstack structure 105, and function as a switchinglayer of MIM 105. CNT material 108 may be deposited by varioustechniques. One technique involves spray- or spin-coating a carbonnanotube suspension over first conductor 102, thereby creating a randomCNT material. Another technique involves growing carbon nanotubes from aseed anchored to the substrate by CVD, PECVD or the like. Discussions ofvarious CNT deposition techniques are found in the '154 application, andrelated U.S. patent application Ser. Nos. 11/968,156, “Memory Cell ThatEmploys A Selectively Fabricated Carbon Nano-Tube ReversibleResistance-Switching Element Formed Over A Bottom Conductor And MethodsOf Forming The Same,” filed Dec. 31, 2007, and 11/968,159, “Memory CellWith Planarized Carbon Nanotube Layer And Methods Of Forming The Same,”filed Dec. 31, 2007, which are hereby incorporated by reference hereinin their entireties for all purposes.

In some embodiments in accordance with this invention, followingdeposition/formation of CNT material 108, an anneal step may beperformed to modify the properties of the CNT material 108. Inparticular, the anneal may be performed in a vacuum or the presence ofone or more forming gases, at a temperature in the range from about 350°C. to about 900° C., for about 30 to about 180 minutes. The annealpreferably is performed in about an 80% (N₂):20% (H₂) mixture of forminggases, at about 625° C. for about one hour.

Suitable forming gases may include one or more of N₂, Ar, and H₂,whereas preferred forming gases may include a mixture having above about75% N₂ or Ar and below about 25% H₂. Alternatively, a vacuum may beused. Suitable temperatures may range from about 350° C. to about 900°C., whereas preferred temperatures may range from about 585° C. to about675° C. Suitable durations may range from about 0.5 hour to about 3hours, whereas preferred durations may range from about 1 hour to about1.5 hours. Suitable pressures may range from about 1 mT to about 760 T,whereas preferred pressures may range from about 300 mT to about 600 mT.

This anneal may be performed prior to the formation of a top electrodeabove CNT material 108. A queue time of preferably about 2 hours betweenthe anneal and the electrode metal deposition preferably accompanies theuse of the anneal. A ramp up duration may range from about 0.2 hours toabout 1.2 hours and preferably is between about 0.5 hours and 0.8 hours.Similarly, a ramp down duration also may range from about 0.2 hours toabout 1.2 hours and preferably is between about 0.5 hours and 0.8 hours.

Although not wanting to be bound by any particular theory, it isbelieved that the CNT material may absorb water from the air and/ormight have one or more functional groups attached to the CNT materialafter the CNT material is formed. Organic functional groups aresometimes required for pre-deposition processing. One of the preferredfunctional groups is a carboxylic group. Likewise, it is believed thatthe moisture and/or organic functional groups may increase thelikelihood of delamination of the CNT material. In addition, it isbelieved that the functional groups may attach to the CNT material, forinstance, during a cleaning and/or filtering process. Thepost-carbon-formation anneal may remove the moisture and/or carboxylicor other functional groups associated with the CNT material. As aresult, in some embodiments, delamination of the CNT material and/or topelectrode material from a substrate is less likely to occur if the CNTmaterial is annealed prior to formation of the top electrode over theCNT material.

Incorporation of such a post-CNT-formation anneal preferably takes intoaccount other layers present on the device that includes the CNTmaterial, inasmuch as these other layers will also be subject to theanneal. For example, the anneal may be omitted or its parameters may beadjusted where the aforementioned preferred anneal parameters woulddamage the other layers. The anneal parameters may be adjusted withinranges that result in the removal of moisture and/or carboxylic or otherfunctional groups without damaging the layers of the annealed device.For instance, the temperature may be adjusted to stay within an overallthermal budget of a device being formed. Likewise, any suitable forminggases, temperatures and/or durations may be used that are appropriatefor a particular device. In general, such an anneal may be used with anyc-based layer or carbon-containing material, such as layers having CNTmaterial, graphite, graphene, amorphous carbon, etc.

In some embodiments in accordance with this invention, followingdeposition/formation of CNT material 108, an optional secondcarbon-based material layer (not shown) may be formed as a protectiveliner covering CNT material 108, such as described in commonly owned,co-pending U.S. patent application Ser. No. 12/415,964, filed Mar. 31,2009, and titled “Electronic Devices Including Carbon-Based Films HavingSidewall Liners, And Methods Of Forming Such Devices” (the “'964application”), which is incorporated by reference herein in its entiretyfor all purposes.

After formation of CNT material 108, an adhesion/barrier layer 110, suchas TiN, TaN, W, WN, tantalum carbon nitride (“TaCN”), or the like, maybe formed over CNT material 108. As shown in FIG. 1, adhesion layer 110may function as a top electrode of MIM device 105 that includes CNTmaterial 108 as the switching layer, and first metal layer 104 andoptional adhesion layer 106 as the bottom electrode. As such, thefollowing sections refer to adhesion/barrier layer 110 as “top electrode110” of MIM 105.

In some embodiments in accordance with this invention, top electrode 110may be deposited using a lower energy deposition technique, e.g., oneinvolving energy levels lower than those used in PVD of similarmaterials. Such exemplary deposition techniques may includenon-conformal deposition, low bias power physical vapor deposition(“LBP-PVD”), low temperature PVD, and other similar techniques. Use of anon-conformal, lower energy deposition technique to deposit topelectrode 110 on the carbon material may reduce the potential fordeposition-associated damage to CNT material 108 and the potential forinfiltration and/or penetration of CNT material 108 by top electrode110. In embodiments foregoing the use of an optional carbon liner, useof lower energy deposition techniques may be particularly advantageousto limit the deleterious effects of the deposition of top electrode 110.Metal deposition techniques that are non-conformal have a lowerlikelihood of depositing metal into a pore in the CNT material 108.

The layerstack of layers 108 and 110 may be patterned, for example, withabout 1 to about 1.5 micron, more preferably about 1.2 to about 1.4micron, of photoresist (“PR”) using standard photolithographictechniques. Thinner PR layers may be used with smaller criticaldimensions and technology nodes. In some embodiments, an oxide hard maskmay be used below the PR layer to improve pattern transfer and protectunderlying layers during etching.

As previously mentioned, CNT material typically has a rough surfacetopography, with pronounced thickness variations that make CNT materialdifficult to etch. Methods in accordance with this invention providemethods of etching CNT material 108 using non-oxygen-based chemistriesthat may be fully compatible with standard semiconductor tooling and/orprocessing equipment. For simplicity, the remaining discussion willrefer to exemplary techniques for etching CNT material 108. Person ofordinary skill in the art will understand that the same techniques maybe used to etch top electrode 110.

In particular, in at least some embodiments of the invention, CNTmaterial 108 may be etched using boron trichloride (BCl₃) and dichlorine(Cl₂) chemistries. For example, CNT material 108 may be etched in aplasma etch chamber using BCl₃ and Cl₂ gas flow inputs, generatingreactive species such as chlorine ions (Cl+) that may be used to etchCNT material 108. The ratio of BCl₃ to Cl₂ may be about 4:1 to about1.8:1, more generally about 70:1 to about 3:5. In at least oneembodiment, an approximately 5:2 ratio of BCl₃ to Cl₂ may be employed toetch CNT material 108 with a substrate bias power of about 100 Watts anda plasma power of about 450 Watts. Exemplary processing conditions for aplasma etch process for etching CNT material 108 are provided below inTable 1. Other ratios, flow rates, chamber pressures, power levels,process temperatures, and/or etch rates may be used.

TABLE 1 EXEMPLARY PLASMA ETCH PROCESS PARAMETERS PREFERRED PROCESSPARAMETER EXEMPLARY RANGE RANGE BCl₃ Flow Rate (sccm) 30-70  45-60 Cl₂Flow Rate (sccm) 0-50 15-25 Pressure (milliTorr) 50-150  80-100Substrate Bias RF (Watts) 50-150  85-110 Plasma RF (Watts) 350-550 390-410 Process Temperature (° C.) 45-75  60-70 Etch Rate (Å/sec) 3-104-5

In accordance with alternative embodiments of this invention, CNTmaterial 108 may be etched using chlorine and argon chemistries. Forexample, CNT material 108 may be etched in a plasma etch chamber usingBCl₃, Cl₂ and argon gas flow inputs, generating reactive species such aschlorine ions (Cl+) and argon ions (Ar+) that may etch a CNT material.The ratio of BCl₃:Cl₂:Ar may be about 4:1:1 to about 1.8:1:1, moregenerally about 70:1:1: to about 3:5:5. In at least one embodiment, anapproximately 5:2:2 ratio of BCl₃:Cl₂:Ar may be employed to etch CNTmaterial 108 with a substrate bias power of about 100 Watts and a plasmapower of about 450 Watts may be used. Exemplary processing conditionsfor a plasma etch process for a CNT material are provided below in Table2. Other ratios, flow rates, chamber pressures, power levels, processtemperatures, and/or etch rates may be used.

TABLE 2 EXEMPLARY PLASMA ETCH PROCESS PARAMETERS PREFERRED PROCESSPARAMETER EXEMPLARY RANGE RANGE BCl₃ Flow Rate (sccm) 30-70 45-60 Cl₂Flow Rate (sccm)  0-50 15-25 Argon Flow Rate (sccm)  0-50 15-25 Pressure(milliTorr)  50-150  80-100 Substrate Bias RF 100-200 125-175 (Watts)Plasma RF (Watts) 350-550 390-410 Process Temperature (° C.) 45-75 60-70Etch Rate (Å/sec) 10-20 13.8-14.5

In some embodiments, top electrode 110 and CNT material 108 may bepatterned using a single etch step. In other embodiments, separate etchsteps may be used. For example, top electrode 110 may be etched using achlorine process (similar to that of Table 1, or Table 2 without theargon flow), and CNT material 108 may be etched using a chlorine-argonchemistry (similar to that of Table 2). In other embodiments, a singleetch procedure may be used (e.g., using a chlorine-argon chemistry as inTable 2). Studies have shown that using argon during the etch mayincrease the etch rate of CNT material 108.

The etch of top electrode 110 and CNT material 108 proceeds down tofirst conductor 102 and exposes gap fill material 111. Such an etchedlayerstack has been observed to have nearly vertical sidewalls 105′ andlittle or no undercut of the CNT material 108.

In some embodiments, prior to etching top electrode 110 and CNT material108, the PR may be ashed using standard procedures. In other embodimentsthe PR is ashed after etching CNT material 108. In embodiments in whichthe PR is ashed, the CNT etch may include about 45-60 sccm of BCl₃,about 15-25 sccm of Cl₂ and about 15-25 sccm of Argon using about125-175 Watts bias for about 55-65 seconds. In embodiments in which thePR is not ashed, the identical conditions may be used with a longer etchtime (e.g., about 60-70 seconds). In either case, a chuck temperature of60-70° C. may be employed during the CNT etch. Exemplary ranges for theCNT dry etch include about 100 to 250 Watts bias, about 45 to 85° C.chuck temperature, and a gas ratio range of about 2:1 to 5:1 BCl₃:Cl₂and about 5:1 Ar:Cl₂ to no argon. The etch time may be proportional tothe CNT thickness.

If ashing is performed after CNT material 108 has been etched, forexample, the bias and/or directionality component of the ashing processmay be increased and the pressure of oxygen during the ashing processmay be reduced. Both attributes may help to reduce undercutting of CNTmaterial 108. Any suitable ashing tool may be used, such as an Iridiaasher available from GaSonics International of San Jose, Calif.

In at least some embodiments, an ashing procedure including two steps isused. Exemplary process conditions for the first ashing step areprovided in Table 3 below. Exemplary process conditions for the secondashing step are provided in Table 4 below. Other flow rates, pressures,RF powers and/or times may be used.

TABLE 3 EXEMPLARY 1^(ST) ASHING STEP PROCESS PARAMETERS PREFERREDPROCESS PARAMETER EXEMPLARY RANGE RANGE CF₄ Flow Rate (sccm) 10-50 20-30N₂H₂ Flow Rate (sccm)  80-120  90-110 H₂O₂ Flow Rate (sccm) 200-350260-290 Pressure (milliTorr) 600-800 650-750 Substrate Bias RF 0 0(Watts) Plasma RF (Watts) 350-450 400-430 Time (seconds)  20-120 50-70

TABLE 4 EXEMPLARY 2^(ND) ASHING STEP PROCESS PARAMETERS PREFERREDPROCESS PARAMETER EXEMPLARY RANGE RANGE O₂ Flow Rate (sccm) 350-450380-420 Pressure (milliTorr) 200-600 380-440 Substrate Bias RF  50-200 90-120 (Watts) Plasma RF (Watts) 350-450 400-430 Time (seconds)  20-12050-70

The bias power may be increased from zero for normal processing, and theashing time may be proportional to the thickness of the PR used.

After the etch of the top electrode 110 and CNT material 108, thelayerstack may be cleaned prior to formation of additional dielectricgap fill 111′. Data indicate that a CNT layer delaminates in EKC typecleans, so after the stack is etched, a dilute hydrofluoric/sulfuricacid clean is performed. Post CNT etch cleaning, whether or not PRashing is performed before CNT etching, may be performed in any suitablecleaning tool, such as a Raider tool, available from Semitool ofKalispell, Mont. Exemplary post-CNT-etch cleaning may include usingultra-dilute sulfuric acid (e.g., about 1.5-1.8 wt %) for about 60seconds and ultra-dilute hydrofluoric (“HF”) acid (e.g., about 0.4-0.6wt %) for 60 seconds. Megasonics may or may not be used. Following suchcleaning, no residual photoresist was observed. If photoresist does comeinto contact with CNT material, the PR is hard to remove, and theelectrical performance of the CNT material suffers.

As such, an exemplary sequence of steps to etch and clean the stack isas follows: (1) pattern the PR; (2) transfer the pattern into an oxidehard mask; (3) ash away the PR (an ash tool may be here because themetal protects the CNT); (4) clean using dilute hydrofluoric andsulfuric acid cleans; (5) etch the stack using BCl₃ and Cl₂ chemistries(no post-etch ashing is used because CNT is exposed); and (6) cleanagain using dilute HF/sulfuric acid cleans. Following the etch and cleansteps, a dielectric sidewall liner may be formed.

After cleaning, deposition of gap fill conventionally would occur.Conventional PECVD techniques, however, for depositing dielectric gapfill material 111′ may employ an oxygen plasma component that is createdin the initial stages of deposition. This initial oxygen plasma may harmCNT material 108, causing undercutting and poor electrical performance.To avoid such harm, methods in accordance with this invention form adielectric sidewall liner 118 to protect sidewalls 105′ of CNT material108 during deposition of the remaining gap-fill dielectric 111′ (e.g.,SiO₂).

Dielectric sidewall liner 118 is deposited using a deposition chemistrythat has a low oxygen content, which produces an “oxygen-poor”dielectric. In one exemplary embodiment, a silicon nitride dielectricsidewall liner 118 followed by a standard PECVD SiO₂ dielectric fill111′ may be used. Whereas stoichiometric silicon nitride is Si₃N₄,silicon nitride (or simply “SiN”) is used herein to refer tostoichiometric and non-stoichiometric silicon nitride alike.

In the embodiment of FIG. 1, dielectric sidewall liner 118 is depositedconformally over the etched layerstack of top electrode/aC/CNT featuresbefore gap fill portion 111′, e.g., the remainder of the dielectric gapfill, is deposited. Dielectric sidewall liner 118 preferably coversouter sidewalls 105′ of CNT material 108 and isolates them fromdielectric fill 111′. If CNT material 108 is overetched, such thatetching of underlying dielectric gap fill material 111 occurs,dielectric sidewall liner 118 may extend below CNT material 108.

In some embodiments, dielectric sidewall liner 118 may comprise about200 to about 500 angstroms of SiN. However, the structure optionally maycomprise other layer thicknesses and/or other materials, such asSi_(x)C_(y)N_(z) and Si_(x)O_(y)N_(z) (with low O content), etc., wherex, y and z are non-zero numbers resulting in stable compounds.

The defined top electrode/CNT features may be isolated with SiO₂ orother dielectric fill 111′, and then planarized to co-expose topelectrode 110, gap fill 111′ and dielectric sidewall liner 118. A secondconductor 112 may be formed over the planar surface, exposing topelectrode 110. Second conductor 112 may include a barrier/adhesion layer114, such as TiN, TaN, WN, Mo, or a similar material, and a metal layer116 (e.g., tungsten or other conductive material).

MIM device 105 may serve as a resistance-switchable memory element formemory cell 100. CNT material 108 may form a resistivity-switchableportion of the memory element of the memory cell, wherein the memoryelement is adapted to switch between two or more resistivity states. Forexample, MIM device 105 may be coupled in series with a steering elementsuch as a diode, a tunnel junction, or a thin film transistor (“TFT”).In at least one embodiment, the steering element may include apolycrystalline vertical diode.

Memory operation is based on a bi-stable resistance change in the CNTstackable layer 108 with the application of high bias voltage (e.g., >4V). Current through the memory cell is modulated by the resistance ofCNT material 108. The memory cell is read at a lower voltage that willnot change the resistance of CNT material 108. In some embodiments, thedifference in resistivities between the two states may be over 100×. Thememory cell may be changed from a “0” to a “1,” for example, with theapplication of high forward bias on the steering element (e.g., adiode). The memory cell may be changed back from a “1” to a “0” with theapplication of a high forward bias. As stated, this integration schemecan be extended to include CNT materials in series with a TFT or tunneljunction as the steering element instead of a vertical pillar diode. TheTFT or tunnel junction steering element may be either planar orvertical.

In accordance with a second exemplary embodiment of this invention,formation of a microelectronic structure includes formation of a diodein series with an MIM device, having a carbon film disposed between abottom electrode and a top electrode, and a dielectric sidewall linerprovided to protect the carbon-based material from degradation during adielectric fill step. The dielectric liner and its use are compatiblewith standard semiconductor tooling.

FIG. 2 is a cross-sectional elevational view of an exemplary memory cellstructure 200 provided in accordance with the present invention. FIGS.2A and 2B depict layers of the memory cell formed in different orders.In FIG. 2A, memory cell structure 200 includes a diode disposed below anMIM device having a dielectric sidewall liner and a CNT film disposedbetween a bottom electrode and a top electrode. In FIG. 2B, memory cellstructure 200′ has the diode disposed above the MIM device.

As shown in FIG. 2A, memory cell structure 200 includes a firstconductor 202 formed over a substrate (not shown). First conductor 202may include a first metal layer 203, such as a W, Cu, Al, Au, or othermetal layer, with a first barrier/adhesion layer 204, such as a TiN, TaNor similar layer, formed over first metal layer 203. First conductor 202may comprise a lower portion of an MIM layerstack structure 205 andfunction as a bottom electrode of MIM 205, as shown in FIG. 2B. Ingeneral, a plurality of first conductors 202 may be provided, e.g.,patterned and etched, and isolated from one another, e.g., by employingSiO₂ or other dielectric material isolation between each of firstconductors 202.

A vertical P-I-N (or N-I-P) diode 206 is formed above first conductor202. For example, diode 206 may include a polycrystalline (e.g.,polysilicon, polygermanium, silicon-germanium alloy, etc.) diode. Diode206 may include a layer 206 n of semiconductor material heavily doped adopant of a first-type (e.g., n-type), a layer 206 i of intrinsic orlightly doped semiconductor material, and a layer 206 p of semiconductormaterial heavily doped a dopant of a second-type (e.g., p-type).Alternatively, the vertical order of diode 206 layers 206 n, 206 i, and206 p may be reversed, analogous to diode 206 shown in FIG. 2B.

In some embodiments, an optional silicide region 206 s may be formedover diode 206. As described in U.S. Pat. No. 7,176,064, which is herebyincorporated by reference herein in its entirety for all purposes,silicide-forming materials, such as titanium and cobalt, react withdeposited silicon during annealing to form a silicide layer. The latticespacings of titanium silicide and cobalt silicide are close to that ofsilicon, and it appears that such silicide layers may serve as“crystallization templates” or “seeds” for adjacent deposited silicon asthe deposited silicon crystallizes (e.g., the silicide layer enhancesthe crystalline structure of diode 206 during annealing). Lowerresistivity silicon thereby is provided. Similar results may be achievedfor silicon-germanium alloy and/or germanium diodes. In some embodimentsusing silicide region 206 s to crystallize diode 206, silicide region206 s may be removed after such crystallization, so that silicon region206 s does not remain in the finished structure.

A TiN or other adhesion/barrier layer or layer stack 207 may be formedabove diode 206. In some embodiments, adhesion/barrier layer 207 maycomprise a layer stack 207 including a first adhesion/barrier layer 207a, a metal layer 207 b, such as of W, and a further adhesion/barrierlayer 207 c, such as of TiN.

In the event that a layerstack 207 is used, layers 207 a and 207 b mayserve as a metal hard mask that may act as a chemical mechanicalplanarization (“CMP”) stop layer and/or etch-stop layer. Such techniquesare disclosed, for example, in U.S. patent application Ser. No.11/444,936, “CONDUCTIVE HARD MASK TO PROTECT PATTERNED FEATURES DURINGTRENCH ETCH,” filed May 31, 2006, which is hereby incorporated byreference herein in its entirety for all purposes. For instance, diode206 and layers 207 a and 207 b may be patterned and etched to formpillars, and dielectric fill material 211 may be formed between thepillars. The stack may then be planarized, such as by CMP or etch-back,to co-expose gap fill 211 and layer 207 b. Layer 207 c may then beformed on layer 207 b. Alternatively, layer 207 c may be patterned andetched along with diode 206 and layers 207 a and 207 b. In someembodiments, layer 207 c may be eliminated, and the CNT material mayinterface directly with metal layer 207 b (e.g., W).

Thereafter, a CNT material 208 may be formed over the adhesion/barrierlayer or layer stack 207 using any suitable CNT formation process (asdescribed previously). Following deposition/formation of CNT material208, an optional second carbon-based material layer (not shown) may beformed as a protective liner covering CNT material 208, as describedabove. Following deposition/formation of CNT material 208, a secondadhesion/barrier layer 210, such as TiN, TaN, WN, Mo, or the like, isformed over CNT material 208.

As shown in FIG. 2A, adhesion layer 207 may function as a bottomelectrode of MIM layerstack 205 that includes CNT material 208 as theswitching layer, and an adhesion layer 210 as a top electrode. As such,the following sections refer to adhesion/barrier layer 207 as “bottomelectrode 207” with respect to FIG. 2A. Similarly, adhesion/barrierlayer 210 is referred to as “top electrode 210” of the MIM 205 of FIG.2A as well as FIG. 2B. Top electrode 210 may be deposited using a lowerenergy deposition technique, as discussed above. An additional hard maskand/or CMP stop layer 214 also may be formed (as shown).

Before formation of a top conductor 212, which may include an adhesionlayer (not shown) and a conductive layer 216, the layerstack may bepatterned and etched, as discussed above in reference to FIG. 1. If anetching process was performed to create the pillars mentioned above,then the etch may apply to layers 208, 210, and possibly 207 c and 214.For example, layers 214, 210 may serve as a hard mask and/or CMP stopfor CNT material 208.

In some embodiments, CNT material 208 may be etched using a differentetch step than the etch step used for second adhesion/barrier layer 210.In other embodiments, a single etch step may be used. Such an etchedfilm stack has been observed to have nearly vertical sidewalls 205′ andlittle or no undercut of CNT material 208. In some embodiments, CNTmaterial 208 may be overetched such that etching of underlyingdielectric gap fill material 211 may occur.

After the etch of the top electrode 210 and CNT material 208, thelayerstack may be cleaned prior to deposition of additional dielectricgap fill 211′. After cleaning, and before deposition of gap fill 211′, adielectric sidewall liner 218 may be formed with an oxygen-poordeposition chemistry (e.g., without a high oxygen plasma component) toprotect the sidewalls 205′ of the CNT material 208 during deposition ofan oxygen-rich gap-fill dielectric 211′ (e.g., SiO₂). The dielectricsidewall liner 218 also may be referred to as a pre-dielectric fillliner.

In the embodiment of FIG. 2, a silicon nitride dielectric sidewall liner218 followed by a standard PECVD SiO₂ dielectric fill 211′ may be used.The silicon nitride dielectric sidewall liner 218 may comprisestoichiometric and/or non-stoichiometric silicon nitride. In someembodiments, the dielectric sidewall liner 218 may comprise about 200 toabout 500 angstroms of SiN. However, the structure optionally maycomprise other layer thicknesses and/or other materials, such asSi_(x)C_(y)N_(z) and Si_(x)O_(y)N_(z) (with low O content), etc., wherex, y and z are non-zero numbers resulting in stable compounds.

The dielectric sidewall liner 218 is deposited conformally over thelayerstack of top electrode/aC/CNT features before gap fill portion211′, e.g., the remainder of the dielectric gap fill, is deposited. Thedielectric sidewall liner 218 preferably covers the outer sidewalls 205′of the CNT material 208 and isolates them from the dielectric fill 211′.In embodiments in which the CNT material 208 is overetched such thatetching of underlying dielectric gap fill material 211 occurs, thedielectric sidewall liner 218 may extend below CNT material 108.

After the defined layerstack of top electrode/CNT features are isolated,with SiO₂ or other dielectric fill 211′, they are planarized toco-expose top electrode 210, gap fill 211′, and SiN dielectric sidewallliner 218. A second conductor 212 is formed over second adhesion/barrierlayer 210, or layer 214, if layer 214 is used as a hard mask and etchedalong with layers 208 and 210. Second conductor 212 may include abarrier/adhesion layer, such as TiN, TaN, WN, or a similar layer, asshown in FIGS. 1 and 2, and a metal layer 216, such as a W or otherconductive layer.

In contrast to FIG. 1, FIG. 2 depicts a layer 214 of tungsten depositedon adhesion/barrier layer 210 before the stack is etched, so that layer214 is etched as well. Layer 214 may act as a metal hard mask to assistin etching the layers beneath it. Insofar as layers 214 and 216 both maybe tungsten, they should adhere to each other well. Optionally, a SiO₂hard mask may be used.

In one exemplary embodiment, a SiN dielectric sidewall liner 218 may beformed, such as described in the '964 application, referenced above.

As shown in FIG. 2B, microelectronic structure 200′ may include thediode 206 positioned above CNT material 208, causing some rearrangementof the other layers. In particular, CNT material 208 may be depositedeither on an adhesion/barrier layer 204, as shown in FIG. 2A, ordirectly on lower conductor 202, as shown in FIG. 2B. Tungsten from alower conductor may assist catalytically in formation of CNT material208, e.g., if grown. Tungsten also appears to adhere well to carbon. Anadhesion/barrier layer 210 may be formed directly on carbon-basedswitching layer 208, followed by formation of diode 206, includingpossible silicide region 206 s. An adhesion/barrier layer 207 may beformed on diode 206 (with or without silicide region 206 s).

FIG. 2B depicts a layer 214, such as tungsten, on layer 207, and layer214 may serve as a metal hard mask and/or adhesion layer to metal layer216 of second conductor 212, preferably also made of tungsten. The stackmay be patterned and etched into a pillar, as described above. In someembodiments, the entire layerstack of layers 206, 207, 208, 210, and 214may be patterned using a single photolithography step.

Above an optional oxide hard mask, or in place thereof, mentioned withrespect to FIG. 1, a layer of silicon may be formed and used as ashrinkable hard mask to further reduce the feature size, e.g., criticaldimension. In some embodiments, the silicon is amorphous as depositedusing a PECVD technique. After patterning the PR layer and etching thepattern into the amorphous silicon to form a silicon hard mask, thesilicon hard mask may be shrunk to reduce the critical dimension of thepattern.

Dielectric sidewall liner 218 may be deposited conformally on the pillarand dielectric fill 211 that isolates first conductors 202. In thiscase, dielectric sidewall liner 218 may extend upward along the entireheight of the layerstack between first conductor 202 and secondconductor 212. Planarization to co-expose the gap fill 211′, metal hardmask layer 214, and dielectric sidewall liner 218 is followed byformation of top conductor 212 to achieve the structure 200′ shown inFIG. 2B.

In accordance with a third exemplary embodiment of this invention,formation of a microelectronic structure includes formation of amonolithic three dimensional memory array including memory cells, eachmemory cell comprising an MIM device having a carbon-based memoryelement disposed between a bottom electrode and a top electrode andcovered by a dielectric sidewall liner. The carbon-based memory elementmay comprise an optional carbon-based protective layer coveringundamaged, or reduced-damage, CNT material that is not penetrated, andpreferably not infiltrated, by the top electrode. The top electrode inthe MIM optionally may be deposited using a lower energy depositiontechnique.

FIG. 3 shows a portion of a memory array 300 of exemplary memory cellsformed according to the third exemplary embodiment of the presentinvention. A first memory level is formed above the substrate, andadditional memory levels may be formed above it. Details regardingmemory array formation are described in the applications incorporated byreference herein, and such arrays may benefit from use of the methodsand structures according to embodiments of the present invention.

As shown in FIG. 3, memory array 300 may include first conductors 310and 310′ that may serve as wordlines or bitlines, respectively; pillars320 and 320′ (each pillar 320, 320′ comprising a memory cell); andsecond conductors 330, that may serve as bitlines or wordlines,respectively. First conductors 310, 310′ are depicted as substantiallyperpendicular to second conductors 330. Memory array 300 may include oneor more memory levels. A first memory level 340 may include thecombination of first conductors 310, pillars 320 and second conductors330, whereas a second memory level 350 may include second conductors330, pillars 320′ and first conductors 310′. Fabrication of such amemory level is described in detail in the applications incorporated byreference herein.

Embodiments of the present invention are useful in formation of amonolithic three dimensional memory array. A monolithic threedimensional memory array is one in which multiple memory levels areformed above a single substrate, such as a wafer, with no interveningsubstrates. The layers forming one memory level are deposited or growndirectly over the layers of an existing level or levels. In contrast,stacked memories have been constructed by forming memory levels onseparate substrates and adhering the memory levels atop each other, asin Leedy, U.S. Pat. No. 5,915,167. The substrates may be thinned orremoved from the memory levels before bonding, but as the memory levelsare initially formed over separate substrates, such memories are nottrue monolithic three dimensional memory arrays.

A related memory is described in Herner et al., U.S. patent applicationSer. No. 10/955,549, “Nonvolatile Memory Cell Without A DielectricAntifuse Having High- And Low-Impedance States,” filed Sep. 29, 2004(hereinafter the '549 application), which is hereby incorporated byreference herein in its entirety for all purposes. The '549 applicationdescribes a monolithic three dimensional memory array includingvertically oriented p-i-n diodes like diode 206 of FIG. 2. As formed,the polysilicon of the p-i-n diode of the '549 application is in ahigh-resistance state. Application of a programming voltage permanentlychanges the nature of the polysilicon, rendering it low-resistance. Itis believed the change is caused by an increase in the degree of orderin the polysilicon, as described more fully in Herner et al., U.S.patent application Ser. No. 11/148,530, “Nonvolatile Memory CellOperating By Increasing Order In Polycrystalline SemiconductorMaterial,” filed Jun. 8, 2005 (the “'530 application”), which isincorporated by reference herein in its entirety for all purposes.

Another related memory is described in Herner et al., U.S. Pat. No.7,285,464, (the “'464 patent”), which is incorporated by referenceherein in its entirety. As described in the '464 patent, it may beadvantageous to reduce the height of the p-i-n diode. A shorter dioderequires a lower programming voltage and decreases the aspect ratio ofthe gaps between adjacent diodes. Very high-aspect ratio gaps aredifficult to fill without voids. A thickness of at least 600 angstromsis preferred for the intrinsic region to reduce current leakage inreverse bias of the diode. Forming a diode having a silicon-poorintrinsic layer above a heavily n-doped layer, the two separated by athin intrinsic capping layer of silicon-germanium, will allow forsharper transitions in the dopant profile, and thus reduce overall diodeheight.

In particular, detailed information regarding fabrication of a similarmemory level is provided in the '549 application and the '464 patent,previously incorporated. More information on fabrication of relatedmemories is provided in Herner et al., U.S. Pat. No. 6,952,030, “AHigh-Density Three-Dimensional Memory Cell,” owned by the assignee ofthe present invention and hereby incorporated by reference herein in itsentirety for all purposes. To avoid obscuring the present invention,this detail will be not be reiterated in this description, but noteaching of these or other incorporated patents or applications isintended to be excluded. It will be understood that the above examplesare non-limiting, and that the details provided herein can be modified,omitted, or augmented while the results fall within the scope of theinvention.

The foregoing description discloses exemplary embodiments of theinvention. Modifications of the above disclosed apparatus and methodsthat fall within the scope of the invention will be readily apparent tothose of ordinary skill in the art. Accordingly, although the presentinvention has been disclosed in connection with exemplary embodiments,it should be understood that other embodiments may fall within thespirit and scope of the invention, as defined by the following claims.

1. A method of etching a carbon nano-tube (“CNT”) film formed over a substrate, the method comprising: coating the substrate with a masking layer; patterning the masking layer; and etching the CNT film through the patterned masking layer using a non-oxygen based chemistry.
 2. The method of claim 1, wherein etching the CNT film includes: loading the substrate into an plasma etch chamber; and etching the substrate using boron trichloride (“BCl₃”) and dichlorine (“Cl₂”).
 3. The method of claim 2, wherein etching the substrate comprises using a ratio of BCl₃ to Cl₂ of about 70:1 to about 3:5.
 4. The method of claim 2, wherein etching the substrate comprises using a ratio of BCl₃ to Cl₂ of about 4:1 to about 1.8:1.
 5. The method of claim 2, wherein etching the substrate comprises using a ratio of BCl₃ to Cl₂ of about 5:2.
 6. The method of claim 2, wherein etching the substrate further comprises using argon (“Ar”).
 7. The method of claim 6, wherein etching the substrate comprises using a ratio of BCl₃ to Cl₂ to Ar of about 70:1:1 to about 3:5:5.
 8. The method of claim 6, wherein etching the substrate comprises using a ratio of BCl₃ to Cl₂ to Ar of about 4:1:1 to about 1.8:1:1.
 9. The method of claim 6, wherein etching the substrate comprises using a ratio of BCl₃ to Cl₂ to Ar of about 5:2:2.
 10. The method of claim 1, wherein the masking layer includes photoresist.
 11. The method of claim 10 further comprising ashing the photoresist.
 12. The method of claim 11, wherein the photoresist is ashed before etching the substrate.
 13. The method of claim 11, wherein the photoresist is ashed after etching the substrate.
 14. The method of claim 11, wherein ashing comprises a two-step ashing procedure.
 15. A memory cell formed using the method of claim
 1. 16. A method of forming a memory cell, the method comprising: forming a layer of a carbon nanotube (“CNT”) material above a substrate; and etching the CNT material in a plasma etch chamber using boron trichloride (BCl₃) and dichlorine (Cl₂), and a substrate bias power of between about 50 and about 150 Watts.
 17. The method of claim 16, wherein the substrate bias power is between about 85 and about 110 Watts.
 18. The method of claim 16, wherein the substrate bias power is about 100 Watts.
 19. The method of claim 16, wherein etching the CNT material comprises using a ratio of BCl₃ to Cl₂ of about 70:1 to about 3:5.
 20. The method of claim 16, wherein etching the substrate comprises using a ratio of BCl₃ to Cl₂ of about 4:1 to about 1.8:1.
 21. The method of claim 16, wherein etching the substrate comprises using a ratio of BCl₃ to Cl₂ of about 5:2.
 22. The method of claim 17, wherein etching the substrate further comprises using argon (“Ar”).
 23. The method of claim 22, wherein etching the substrate comprises using a ratio of BCl₃ to Cl₂ to Ar of about 70:1:1 to about 3:5:5.
 24. The method of claim 22, wherein etching the substrate comprises using a ratio of BCl₃ to Cl₂ to Ar of about 4:1:1 to about 1.8:1:1.
 25. The method of claim 22, wherein etching the substrate comprises using a ratio of BCl₃ to Cl₂ to Ar of about 5:2:2.
 26. The method of claim 17, further comprising forming a steering element coupled to the CNT layer.
 27. The method of claim 26, wherein the steering element comprises a diode.
 28. The method of claim 27, wherein forming the steering element comprises: forming one or more layers of silicon above the substrate; and etching the one or more layers of silicon.
 29. The method of claim 28, comprising etching the one or more layers of silicon and the CNT layer in a single etching step.
 30. The method of claim 28, comprising etching the one or more layers of silicon and the CNT layer separately.
 31. A memory cell formed using the method of claim
 17. 32. A method of forming a memory cell, the method comprising: forming a layer of a carbon nanotube (“CNT”) material above a substrate; and using a non-oxygen based chemistry to etch the CNT material to have nearly vertical sidewalls and little or no undercut of the CNT material.
 33. The method of claim 32, further comprising forming a steering element coupled to the CNT layer.
 34. The method of claim 33, wherein the steering element comprises a diode.
 35. The method of claim 34, wherein forming the steering element comprises: forming one or more layers of silicon above the substrate; and etching the one or more layers of silicon.
 36. The method of claim 35, comprising etching the one or more layers of silicon and the CNT layer in a single etching step.
 37. The method of claim 35, comprising etching the one or more layers of silicon and the CNT layer separately.
 38. A memory cell formed using the method of claim
 32. 